This invention is generally directed to an electrophotographic imaging system and more specifically to a method of imaging utilizing an improved overcoated electrophotographic imaging member.
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known, one of the most widely used processes being xerography. The art of xerography is described in C. F. Carlson U.S. Pat. No. 2,297,691 and involves the formation of an electrostatic latent image on the surface of a photosensitive plate which is usually referred to as the photoreceptor. The photoreceptor itself comprises a conductive substrate containing on its surface a layer of photoconductive insulating material; and in many instances there can be used a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection from the substrate into the photoconductive layer upon changing of the plate surface since if charge injection were allowed this would adversely affect the quality of the resulting image.
The plate is charged in the dark, for example, by exposing it to an imagewise pattern of activating electromagnetic radiation. The light struck areas of the imaging layer are rendered relatively conductive and the electrostatic charge is selectively dissipated in those irradiated areas. Subsequent to exposing the photoconductor the electrostatic latent image on this image bearing surface is rendered visible with a finely-divided colored marking material known as toner. This toner is attracted principly to those areas of the image bearing surface which retain the electrostatic charge thereby forming a visible powder image. The transfer of the toner image to a receiver member such as paper with subsequent fusing of the toner into the paper provides a permanent copy. The imaging surface of the photoreceptor can then be cleaned by any of several known methods including blade cleaning, the purpose of the cleaning generally being to remove any residual toner. The electrostatic latent image can also be used in a number of other ways as, for example, electrostatic scanning systems may be employed to read the latent image or the latent image may be transferred to other materials by TESI techniques and stored. The developed image can then be read or permanently affixed to the photoconductor when the imaging layer is not to be reused.
Numerous types of photoreceptors can be used in the above described method and are well known such photoreceptors including organic materials, inorganic materials and mixtures thereof. There are known photoreceptors wherein the charge carrier generation and charge carrier transport functions are accomplished by discrete contiguous layers. Also known are photoreceptors which include an overcoating layer of an electrically insulating polymeric material and in conjunction with this overcoated type photoreceptor there have been proposed a number of imaging methods. However, the art of xerography continues to advance and more stringent demands need to be met by the copying apparatus in order to increase performance standards, to obtain higher quality images and to act as protection for the photoreceptor. In the present invention there is described an electrophotographic imaging method employing an improved overcoated electrophotographic imaging member.
U.S. Pat. No. 3,041,167 teaches an electrophotographic imaging method which employs an overcoated imaging member comprising a conductive substrate, a photoconductive insulating layer and an overcoating layer of an electrically insulating polymeric material. This member is utilized in an electrophotographic copying method by, for example, initially charging the member with an electrostatic charge of a first polarity and imagewise exposing to form an electrostatic latent image which can then be developed to form a visible image. The visible image is transferred to receiver member and the surface of the imaging member is cleaned to complete the imaging cycle. Prior to each succeeding cycle the imaging member can be charged with an electrostatic charge of a second polarity which is opposite in polarity to the first polarity. Sufficient additional charges of the second polarity are applied so as to create across the member a net electrical field of the second polarity. Simultaneously, mobile charges of the first polarity are created in the photoconductive layer such as by applying an electrical potential to the conductive substrate. The imaging potential which is developed to form the visible image is present across the photoconductive layer and the overcoating layer.
Various imaging methods can be used such as those described by Mark, in an article appearing in "Photographic Science and Engineering," Volume 18, No. 3, pages 254-261, May/June, 1974. The process referred to by Mark as the Katsuragawa and Canon processes can basically be divided into four steps. The first is to charge the insulating overcoating. This is normally accomplished by exposing it to d.c. corona of a polarity opposite to that of the majority charge carrier. When applying a positive charge to the surface of the insulating layer, as in the case where an n-type photoconductor is employed, a negative charge is induced in the conductive substrate, injected into the photoconductor and transported to and trapped at the insulating layer-photoconductive layer interface resulting in an initial potential being solely across the insulating layer. The charged plate is then exposed to a light and shadow pattern while simultaneously applying to its surface an electronic field of either alternating current (Canon) or direct current of polarity opposite that of the initial electrostatic charge (Katsuragawa). The plate is then uniformly exposed to activating radiation to produce a developable image with potential across the insulating overcoating and simultaneously reduce the potential across the photoconductive layer to zero. In other processes described in the Mark article, i.e., the Hall and Butterfield processes, the polarity of the initial voltage is the same sign as the majority charge carrier and reverse polarity is encountered during erase.
In processes where the voltage must initially be placed across the overcoating, for example, in step 1 of the Canon process, either an electron injecting contact for the majority carrier or the ability to bulk generate carriers or an ambipolar photoconducting layer must be used. In processes where the initial voltage polarity is the opposite sign of the majority carrier, there is required an injecting contact for the majority carrier, the ability to bulk generate carriers or an ambipolar photoconducting layer.